Semiconductor devices, especially power semiconductor devices such as power semiconductor switches, often include a current and temperature measurement functionality for detecting faulty or undesired modes of operation during which undesirably high currents or temperatures occur. Such faulty or undesired modes of operation may be, inter alia, an over-load, or a short circuit.
Power semiconductor switches capable of detecting over-temperature, over-load, short-circuits, etc. are often referred to as “smart switches”. Typically such smart switches include at least one power transistor (e.g. a DMOS high-side switch) and an over-current detection circuit for each power transistor that compares a measured signal that represents the load current flowing through the transistor with a defined threshold value that represent a maximum current. When the load current reaches or exceeds the maximum current, the load current is switched off in order to protect the device.
However, in many applications smart switches have to handle high inrush currents. This may be the case, e.g., when switching on incandescent lamps, electric motors or the like. The inrush currents are typically much higher than the maximum current, yet the high inrush currents are transient and usually do not cause a dangerous over-temperature. However, the over-current protection circuit included in the smart switch needs to distinguish between high inrush currents and over-currents resulting from a short circuit. For this reason the threshold, which determines the maximum current, is set to a higher value (higher than during normal operation) during a start-up phase in which transient inrush currents may occur. This start-up phase is usually defined as a fixed time interval, e.g., 10 ms. When this time interval has elapsed, the threshold, which determines the maximum current, is reset to the lower—nominal—value.
When an over-current is detected (i.e. when an over-current event occurs) the device may be deactivated. That is, the device is latched in an inactive state in which the load current is switched off. However, to avoid a deactivation during the start-up phase the device is re-activated after an over-current event for a defined number of times (e.g. 32 times). That is, the device is finally deactivated (and not re-activated) when the maximum number of over-current events occurs during the start-up phase. After the start-up phase, a single over-current event is sufficient for latching the device in an off-state.
The “switching” between the start-up phase with a high maximum current threshold and the normal operation with a low maximum current threshold is usually implemented digitally with a finite state machine (FSM). A further problem arises when the smart switch is supplied via a long supply line. For example, in automotive applications the supply line may be up to 5 meters long or even longer, resulting in a line resistance of about 100 mΩ and a line inductance of about μH. As a consequence the voltage drop across the supply line may be rather high due to the high inrush currents during the start-up phase. In fact, the voltage drop across the supply line may be high enough to trigger the under-voltage detection. When an under-voltage is detected (i.e. in case of an under-voltage event) the state-machine and thus the counter, which counts the over-current events during the start-up phase, is reset. As a consequence, the over-current event counter will never reach the maximum number as the supply voltage drops and thus the counter is reset every time the switch is closed and the inrush current starts to rise.
The problem to be solved by the present invention is to provide a semiconductor device including an over-current protection which can handle transient voltage drops across the supply line.